1. Field of the Invention
The present invention relates to strain gauges. More specifically, the present invention relates to an improved inductance strain gauge usable for measuring nonelectric variables such as strain of a material being stress-tested.
2. Prior Art
Known resistance strain gauges have wire or foil grids bonded to or embedded in a substrate. The substrate is attached to a material to be stress-tested. When the wire grid elongates or is compressed due to the applied stress, the resistance of the wire or foil grid changes. Use of such gauges is described in Mark's Standard Handbook for Mechanical Engineers, Ninth Edition, McGraw-Hill Book Company, New York, 1987, at pages 5-58 to 5-59 as follows:
Gages must be properly selected in accordance with manufacturer's recommendations. The surface to which the gage is applied must be clean, the proper cement must be used, and the gage assembly must be coated for protection against environmental conditions (e.g., moisture). PA0 A gaging unit, usually a Wheatstone bridge or a ballast circuit . . . is needed to detect the signal resulting from the change in resistance of the strain gage. The strain and, therefore, the signal are often too small for direct handling, so that amplification is needed, with a metering discriminator for magnitude evaluation. PA0 The signal is read or recorded by a galvanometer, oscilloscope, or other device. Equipment specifically constructed for stain measurement is available to indicate or record the signal directly in strain units. PA0 Static strains are best gaged on a Wheatstone bridge, with strain gages wired to it . . . . With the bridge set so that the only balance is the change of resistance in the active-strain gage, the potential difference between the output terminals becomes a measurement of strain. Since the gage is sensitive to temperature as well as strain, it will measure the combined effect. However, if a "dummy" gage, cemented to an unstressed piece of the same metal subjected to the same climatic conditions, is wired into the bridge leg adjacent to the one containing the "active" gage, the electric-resistance temperature effect is canceled out. Thus the active gage reports only that which is taking place in the stressed plate. The power supply can be either ac or dc. PA0 It is sometimes useful to make both gages active--e.g., mounted on opposite sides of a beam, with one gage subjected to tension and the other to compression. Temperature effects are still compensated, but the bridge output is doubled. In other instances, it may be desirable to make all four bridge arms active gages. The experimenter must determine the most practical arrangement for the problem at hand and must bear in mind that the bridge unbalances in proportion to the difference in the strains of gages located in adjacent legs and to the sum of strain in gages located in opposite legs.
When the deformation is large or the specimen is cracked, the resistance wire or foil may be broken so that the stress test cannot be completed. For those circumstances in which a resistance strain gauge is appropriate, there still is the requirement for complicated and expensive monitoring equipment such as high-power amplifier, metering discriminator, galvanometer, oscilloscope, etc. As noted in the passage quoted above, care must be taken to guard against moisture. In addition, the electrical cable connected to the resistance strain gauge must not be too long and, in the case of an AC power supply to the resistance strain gauge, the frequency must be chosen carefully. All in all, careful planning and highly skilled technicians are required if reliable measurements are to be made.
Known inductance strain gauges have thousands of coils wound in ten or more layers and an enclosing shield of magnetic conductive material. The exciting frequency of the AC supply can be several kilohertz to 20 kilohertz. The load impedance of the known inductance strain gauge must be ten times or more than the output impedance of the coils. The supply current must be small to ensure linearity and stability of response. The connecting cable should be short. The overall size of the known inductance strain gauge is large so that it cannot be applied directly to a specimen, particularly if the specimen is small.